1. Field of the Invention
The present invention relates to a spectroscopic detector, and more particularly, to a spectroscopic detector that simultaneously detects light of a wide wavelength range.
2. Description of the Related Art
A spectroscopic detector is basically made up of a spectroscopic element that disperses light, a condensing optical system that condenses the dispersed light per wavelength and a photodetector that detects the dispersed light. Such a spectroscopic detector identifies the wavelength of the light detected by the photodetector from coordinates at which the dispersed light impinges on the photodetector, to be more precise, coordinates at which the dispersed light impinges on a wavelength calculation surface.
The wavelength calculation surface is generally provided on an image forming surface of the condensing optical system. When a single channel detector is used as the photodetector, the surface on which a slit for limiting light incident on the photodetector is arranged becomes the wavelength calculation surface. On the other hand, when an array detector made up of a plurality of cells is used as the photodetector, light-receiving surfaces of the cells become the wavelength calculation surfaces.
Normally, the size of one step that the slit can move on the wavelength calculation surface or the size of each cell on the wavelength calculation surface is constant. For this reason, coordinates at which light impinges on the wavelength calculation surface preferably linearly changes with respect to the wavelength, and actually many photodetection apparatuses calculate wavelengths assuming that coordinates at which light impinges on the wavelength calculation surface have a linear relationship with the wavelength.
However, no linear relationship actually holds between coordinates at which light impinges on the wavelength calculation surface and the wavelength.
Interference conditions of a diffraction grating, which is a spectroscopic element, is generally expressed by the following Equation (1), where, α is an angle of incidence upon the diffraction grating, β is an angle of emergence from the diffraction grating, λ is an incident wavelength, N is a grating frequency of the diffraction grating and λ is a diffraction order.sin α+sin β=Nmλ  (1)
Assuming that the angle of incidence α is constant, when both sides of Equation (1) are differentiated by the angle of emergence β, the following Equation (2) is derived. Here, dβ/dλ represents a variance of the angle of emergence.dβ/dλ=Nm/cos β  (2)
Since the variance of the angle of emergence depends on cos β, the angle of emergence nonlinearly changes with respect to the wavelength. That is, the diffraction grating causes the dispersed light to exit at a nonlinear angle of emergence with respect to the wavelength.
In consideration of the fact that a lens whose chromatic aberration is optimally compensated or a condensing mirror free of chromatic aberration is normally used as the condensing optical system, nonlinearity of the angle of emergence with respect to the wavelength is converted to nonlinearity of the condensing position (coordinates) with respect to the wavelength by the condensing optical system.
Furthermore, since the variance of the angle of emergence is inversely proportional to cos β, the diffraction grating has a characteristic that the greater the distance of the angle of emergence β from 0, the stronger the nonlinearity of angle of emergence β with respect to the wavelength λ. On the other hand, it is not possible to achieve the angle of emergence β=0 with an arrangement where it is easy to obtain high diffraction efficiency with the diffraction grating, Littrow arrangement with a blazed diffraction grating or arrangement that satisfies a Bragg condition with a VPH (Volume Phase Holographic) grating in particular. For this reason, in a general environment in which a diffraction grating is used, nonlinearity becomes conspicuous and influences thereof are not negligible.
As described so far, with the conventional spectroscopic detector, no linear relationship holds between coordinates at which light impinges on the wavelength calculation surface and the wavelength because of nonlinearity of the angle of emergence with respect to the wavelength of the diffraction grating, which is a spectroscopic element. For this reason, since there is a deviation (hereinafter described as “detected wavelength deviation”) between the wavelength calculated on the assumption that coordinates and the wavelength have a linear relationship (hereinafter described as “calculated wavelength”) and the wavelength of light actually detected by a photodetector (hereinafter described as “incident wavelength”), it is difficult to accurately detect the wavelength.
FIG. 1 is a schematic diagram illustrating a configuration of a spectroscopic detector according to a prior art. The amount of specific deviation in detected wavelength generated in a spectroscopic detector will be described with reference to FIG. 1 hereinafter.
A spectroscopic detector 100a illustrated in FIG. 1 includes a diffraction grating 1 that disperses incident light IL, a photodetector 2 that detects the dispersed light and a condensing optical system 3a disposed between the diffraction grating 1 and the photodetector 2 to condense the light dispersed by the diffraction grating 1 to the photodetector 2.
The spectroscopic detector 100a is designed on the assumption that the y-coordinate at which light impinges on a wavelength calculation surface 2a and the wavelength have a linear relationship, so that a central value λC in a maximum range of wavelengths of light that can be simultaneously captured is 600 nm, width Δλ of the maximum wavelength range is 200 nm and the maximum wavelength range λC−Δλ/2 to λC+Δλ/2 is 500 nm to 700 nm. The maximum wavelength range is determined by a variance of the diffraction grating 1, a focal length of the condensing optical system 3a and the magnitude of the photodetector 2.
The diffraction grating 1 is a blazed diffraction grating having a grating frequency of 800 lines/mm and is set up according to Littrow mounting. Furthermore, the condensing optical system 3a is a lens whose chromatic aberration is optimally corrected and disposed so that the optical axis thereof is substantially parallel to light G having the center wavelength λC.
In the spectroscopic detector 100a configured as shown above, the angle of emergence from the diffraction grating 1 is nonlinear with respect to the wavelength and the angle of emergence β increases as the wavelength increases. As a result, the angle variance also increases as the wavelength increases as shown in aforementioned Equation (2). Furthermore, since the chromatic aberration of the condensing optical system 3a is optimally corrected, the nonlinearity of the angle of emergence with respect to the wavelength is also reflected in coordinates at which light impinges on the photodetector 2 as is.
Therefore, when coordinates of incident light of each wavelength is calculated assuming the linearity of the variance characteristic using the variance of light G of 600 nm as a reference, light B of 500 nm impinges inside the calculated coordinates and light R of 700 nm impinges outside. That is, both light B and light G are calculated as having larger wavelengths than the incident wavelength. To be more specific, light B and light R are miscalculated by approximately 1 nm as 501 nm and 701 nm respectively.
As a result, the spectroscopic detector 100a actually detect light as having 499 nm to 699 nm, deviated by approximately 1 nm from light of 500 nm to 700 nm. That is, a deviation is produced between the incident wavelength and the calculated wavelength.
Assuming the y-coordinate at which light G impinges on the photodetector 2 (wavelength calculation surface 2a) is 0 (origin), yB is a displacement from the origin of the y-coordinate to the coordinate at which light B impinges on the photodetector 2, yR is a displacement from the origin of the y-coordinate to the coordinate at which light R impinges on the photodetector 2, θB is an angle formed by light G and light B and θR is an angle formed by light G and light R, a relationship of yB<yR and θB<θR holds in the spectroscopic detector 100a as shown in FIG. 1.
A technique effective for solving such a technical problem that a detected wavelength deviation occurs in the spectroscopic detector is disclosed in Japanese Patent Laid-Open No. 2000-304614 and Japanese Patent Laid-Open No. 9-89668.
Japanese Patent Laid-Open No. 2000-304614 discloses a technique of arranging nonlinear dispersion compensation means between a wavelength dispersion element that is a diffraction grating or the like and a condensing optical system and thereby compensating nonlinearity of the angle of emergence of a wavelength dispersion element. To be more specific, the document discloses a technique of inserting a prism between a diffraction grating and a condensing optical system to compensate nonlinearity of the angle of emergence and thereby flattening a wavelength dispersion characteristic. Furthermore, an example is also disclosed where a configuration uniting a diffraction grating and a prism eliminates the necessity for adjusting relative positions between the diffraction grating and the prism.
Japanese Patent Laid-Open No. 9-89668 discloses a technique of arranging the normal of a diffraction grating and the optical axis of a condensing optical system parallel to each other using an f·sin θ lens for the condensing optical system and thereby resolving nonlinearity of an image height (incident coordinate) with respect to the wavelength.
According to the techniques disclosed in Japanese Patent Laid-Open No. 2000-304614 and Japanese Patent Laid-Open No. 9-89668, it is possible to compensate for a deviation in a detected wavelength deriving from nonlinearity of the angle of emergence from the diffraction grating or the like.